An Overview of the Troilus Au-Cu Deposit, Frotet-Evans Greenstone Belt, Opatica Subprovince, Quebec
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The Troilus Au-Cu deposit is a low-grade, high-tonnage deposit, located in the eastern part of the Archean Frotet-Evans greenstone belt, in the Opatica Subprovince, Superior Province, Quebec. Total past production and current combined indicated and inferred resources were estimated over 6.4 Moz of contained gold (or 200 t Au, in approximately 230 Mt of ore grading 0.87 g/t Au, Troilus Gold Corp., 2019). The deposit comprises two main orebodies, the 87- and the J-zones. Both are hosted in ca. 2791 Ma calc-alkaline intrusive rocks, consisting of a metadioritic stock, an amphibolitic unit, and a magmatic or magmatic-hydrothermal breccia. These rocks are crosscut by ca. 2782 Ma felsic dikes. The calc-alkaline suite was emplaced into the volcano-sedimentary sequence of the Troilus Group. The Troilus deposit is hosted in a zone of intense deformation and experienced upper-greenschist to lower-amphibolite metamorphic conditions. The orebodies in the Troilus deposit display an anastomosed pattern, being parallel to subparallel to a penetrative NE-trending foliation, moderately to steeply-dipping to NW. The amphibolite, the brecciated unit, and the felsic dikes represent the main host rocks for the mineralization, which has been broadly divided in two distinct types: (i) Au-Cu in fine-grained chalcopyrite-pyrrhotite-pyrite disseminations and stringers parallel to the foliation, representing the dominant ore in the Troilus deposit. It occurs in association with a strong biotitic alteration zone, and it is restricted to the amphibolite and to the matrix of the breccia, in a corridor zone between two main felsic dikes; and (ii) Au-bearing quartz-veins and veinlets in association with sericitic alteration and silicification in secondary high-strain zones, occurring in amphibolite, breccia and felsic dikes. The Troilus Au-Cu deposit has been interpreted to be formed similarly to porphyry Au-Cu deposits, based on the disseminated style of the mineralization in association with a zoned hydrothermal alteration assemblage, the presence of multi-stage felsic porphyry intrusions and an in-situ hydrothermal breccia. Some studies have suggested a multi-stage structurally controlled “orogenic” gold type model, based on the recognition of two styles of mineralization and their restriction to superimposed distinct and unrelated structures. However, the difficulties imposed by the intense deformation that occurred in the deposit region, associated with a lack of robust structural, geochemical, petrological, and mineralogical data from the deposit- to district-scale have resulted in a limited understanding of the ore system at Troilus. This report presents a literature review, summarizes and discusses relevant points and controversies regarding the previous studies, while addressing future recommendations to achieve a better understanding of ore genesis at the Troilus Au-Cu deposit.